There is conventionally known a technique of displaying an image of surroundings of a mobile object, such as a vehicle, that is captured by an onboard camera mounted on the mobile object, together with an image serving as a drive assisting indicator superimposed thereon. To superimpose such an indicator on the image of surroundings, a steering angle detected by a steering angle sensor or the like is used. The detected steering angle sometimes differs from the actual steering angle due to long use, steering replacement, and the like, and therefore, a steering angle neutral point is corrected (Patent Literature 1, for example).